Every year many persons are involved in traffic-related accidents resulting in injury or death. In order to reduce the number of accidents and/or to reduce their impact, vehicle safety systems have been developed.
Protective safety systems, such as bumpers, seat belts, crumple zones and air-bags have been in use for many years. They work by dispersing the kinetic energy resulting from a collision in order to protect the occupants of the vehicle.
In addition to the protective safety systems, another type of safety systems has been introduced in the recent years. These in-vehicle systems are designed to help the driver to prevent, avoid or mitigate an impending collision or accident. They are therefore called preventive safety systems. Advanced preventive safety systems work by not only sensing the motion of vehicle hosting the system but also perceiving the traffic situation around the vehicle. They are able to inform the driver of possible danger, to warn the driver of an impending accident, and to detect and classify dangerous situations. In addition, the system may actively assist or intervene in the operation of the vehicle in order to avoid the accident or mitigate its consequences.
A collision avoidance system is an example of a preventive safety system. If a collision is likely to occur and the system detects that the driver does not react in an appropriate manner, the system may be adapted to apply an emergency operation and/or to warn the driver. For example, when the system determines that a collision is unavoidable, it may apply an autonomous brake to the vehicle. If the speed is too high to be able to avoid the obstacle ahead, e.g. a lead vehicle, the system may at least help to lower the speed in order to reduce the consequences of the collision. As described herein, the term collision avoidance system is used for systems adapted to avoid collisions as well as systems mitigating the consequences of a collision. Often, the difference between whether a collision is avoided or not, depends on the speed of the host vehicle and the relative speed to the obstacle.
The term host vehicle is herein used to refer to the own vehicle, which hosts a collision avoidance system. The term lead vehicle is used to refer to the vehicle located closest to, and in front of the host vehicle. Normally the lead vehicle is driving in essentially the same direction as the host vehicle.
In general, a collision avoidance system comprises three separate subsystems: a perception system, a detection system and an action system. The perception system comprises different sensors, which are used to determine the motion of the host vehicle and to perceive the traffic situation therearound. The detection system uses the output of the perception system in order to detect possible adverse situations and make a decision to inform, warn or intervene. The action system executes the action or actions decided by the detection system.
The execution of a possible intervention is a trade-off between effectiveness of the collision avoidance system and the risk of having a false alarm. It is for example undesirable that the collision avoidance system brakes the host vehicle without there actually being a real emergency situation.
In rush-hour-traffic, the traffic is often dense and the time-gap between vehicles can be small. If the driver of a lead vehicle for some reason collides with another external object, such as a third vehicle being in front of the lead vehicle, it can be difficult for the driver of the host vehicle to avoid a collision without assistance, since, due to the collision, the lead vehicle very suddenly and unpredictably changes its motion. There is therefore very little time for the driver of the host vehicle to react to the sudden change in the traffic situation and make an appropriate decision in order to avoid and/or mitigate a collision with the lead vehicle. As mentioned above, a collision avoidance system according to known technology could help the driver of the host vehicle by intervening in such a situation. However, due to the trade-off of the execution between effectiveness and the desire to avoid false interventions mentioned above, the host vehicle may come quite close to the lead vehicle before the emergency operation is initiated. The host vehicle may come so close that a collision with the lead vehicle cannot be avoided.
The document EP 2 172 920 A1 discloses an onboard warning system for detecting a presence of external objects, e.g. other vehicles, within a detection area. The system measures the acceleration of the external object and reacts if it exceeds a threshold value. This may happen when the external vehicle performs an emergency braking and/or steering manoeuvre. A warning signal is then generated.
However, even if a collision involving the lead vehicle sometimes is preceded by a rapid change of acceleration of the lead vehicle, such as a hard braking, there are also many cases, in which a collision involving the lead vehicle occurs without being preceded by a rapid change of acceleration of the lead vehicle, or without the rapid change of acceleration being observed, e.g. due to a sensor being obscured by another object. The lead vehicle itself may be hit by another vehicle and suddenly be thrown in an unpredicted direction. Another case would be that the lead vehicle collides with a pedestrian or an animal, which are rather light objects in comparison to the vehicle, not changing the acceleration of the lead vehicle very much.
It is therefore desired to provide an improved collision avoidance system, which can better handle a situation than an existing collision avoidance system when a collision involving a first external object occurs in the vicinity of the host vehicle. It is further desirable to provide a method suitable for reducing the risk of a collision between the host vehicle and the first external object in such a situation.